Neurotoxicity is a common dose limiting or significant side effect of many chemotherapeutic agents emphasizing the importance of assessing neurotoxicity in the screening of novel chemotherapeutic agents. Current methods of screening for neurotoxicity typically utilize morphologic endpoints. The cost and resource requirements of these methods limits the number of chemicals that can be evaluated and the utility of these assessments is diminished by their inherent sensitivity and difficulty in obtaining quantitative data. Previous work has demonstrated up regulation of several genes following toxic insults and physical injury to the peripheral nervous system. The low affinity nerve growth factor receptor (p75) has been most extensively characterized and has exhibited the greatest increases and longest duration of elevated expression after injury. The ability to detect elevations in mRNA prior to the onset of structural changes together with the ability to assess changes along the entire axon suggest that up regulation of p75 may provide a more sensitive index of neurotoxicity than morphological endpoints. The proposed project is based upon the hypothesis that elevated p75 expression in the peripheral nervous system is a general response to toxic insult that is quantifiable and precedes the onset of morphological changes. This hypothesis will be tested through exposing rats to neurotoxic agents that selectively target the neuron, axon or Schwann cell and then determining if elevated p75 transcription precedes the onset of structural changes at the light and electron microscope level. The quantity of p75 mRNA will be determined in peripheral nerve using real time RT-PCR and the levels of p75 protein in nerve evaluated using immunohistochemistry. The utility of measuring truncated p75 in plasma and urine to I detect neurotoxtcity will also he determined by ELISA and plasmon resonance. Morphologic assessments will he used to determine the temporal relationship of p75 expression to the onset of structural changes. The significance of these investigations lies in the development of more efficient and sensitive methods for detecting neurotoxicity of new therapeutic agents, monitoring neurotoxicity in patients during therapy, defining bioactivation pathways of neurotoxic agents and assessing the neurotoxicity of environmental compounds. The benefits of these methods can be realized through facilitating development of new therapeutic agents and recognizing neurotoxic chemicals before the occurrence of human neurotoxicity.